The present invention relates to a device and method for forming an improved conductive interconnection between a voltage plane on the back side of a printed circuit card and a thick metal heat sink.
Many modern high power radio frequency or microwave applications require significant current carrying capability and/or significant thermal dissipation. To meet these needs, a thick metal backer ("TMB") is often connected to an external ground plane located on the surface of a printed circuit board ("PCB") to improve the electrical performance of the ground plane and also to provide a heat sink for thermal dissipation. Current methods for connecting such a thick metal back plate include sweat soldering, non-conductive adhesive bonding followed by plating through holes for electrical interconnection, gold coating the ground plane and applying a silicone adhesive which includes a mixture of silver particles, or by mechanical interconnection such as screws, rivets or soldered pins. Each of these methods have proven less than ideal as they tend to be too costly while compromising performance. Also, some of these methods result in poor reliability in the resulting circuit board assembly.
The sweat solder method entails soldering a conductive ground plane located on the back of a PCB onto a TMB. The resulting electrical performance is acceptable, because a highly conductive interface is formed. This interface allows for conductive interconnection across the entire ground plane. The reliability is less than ideal, however, due to the low compliance of the soldered interconnection. This low compliance results in the interconnection being unable to accommodate mechanical stress caused by the mismatch of the coefficient of thermal expansion of the PCB compared to that of the ground plane. Also, such sweat soldering is a complex and costly process prone to defects. Defects can occur in the bonding because the bond may reflow during subsequent processing.
Non-conductive adhesive or fusion bonding of the PCB to the TMB also is not ideal due to prohibitive cost, unreliable performance and complex manufacturing. Often, raw dielectric materials, for example polytetrafluoroethylene microwave laminates, are pre-bonded to a TMB of copper, brass or aluminum. The TMB is then discretely interconnected with the PCB using plated through holes or mechanical connecting means such as pressure-fitted or soldered pins. Such discrete electrical interconnects are less desirable than an area electrical interconnection due to compromised localized grounding.
Simple non-adhesive mechanical interconnection such as screws or rivets may also be used to join the PCB and TMB together but imperfect coplanarity between the PCB and TMB, coupled with localized thermal stresses tends to induce localized areas of non-contact, preventing global interconnection and changing the contact resistance over time. Also, resistance stability across the interface is less than desirable and ultimately may lead to failure of the device.
The mechanical and electrical stabilities of conductively-bonded interfaces between the ground plane on a PCB and the TMB have proven difficult to control. Typically, a TMB is formed of copper, brass or aluminum depending upon the need for electrical and thermal conductivity, weight, and ease of machining versus economic considerations. The bonding surface on the PCB is often provided with a copper, tin-lead, tin or gold finish. The TMB is typically aluminum, brass or copper. With the exception of gold, these metals are prone to oxidative, hydrolytic and corrosive processes. The processes lead to the formation of metal oxides, hydroxides and other corrosive products at the interface between the conductive adhesive and metal adherend which ultimately can compromise both the electrical and mechanical stability of the bonding and eventually, the performance and reliability of the packaging structure. This problem is particularly troublesome in humid environments, especially in the case of adhesively bonded aluminum.
Aluminum surfaces are normally protected by a thin layer of aluminum oxide that provides passivation of the metal at room temperature and moderate relative humidity. Although the native oxide of aluminum is a poor conductor, it is thin enough to allow a reasonably low contact resistance for conductive interconnections and typically resists further degradation of the electrical interconnection. However, experience has shown that PCB's conductively bonded to aluminum and exposed to higher values of temperature and humidity induce a transformation of the aluminum oxide (Al.sub.2 O.sub.3) to aluminum oxyhydroxide (AlOOH) and finally, if the reaction is complete, aluminum hydroxide (Al(OH).sub.3). Aluminum hydroxide is mechanically weak, non-conductive and non-passivating (i.e., offers no further protection of the underlying aluminum/aluminum oxide from corrosion). This aluminum hydroxide can build up to significant thicknesses which can result in significant increases in interfacial resistance through the bond and to mechanical separation of the bonded surface. The rate of formation of the aluminum hydroxide is much greater under a conductive bonding adhesive, such as silver-filled epoxy, than for exposed aluminum, perhaps due to corrosive species in the adhesive resin, and/or to galvanic coupling between the silver in the adhesive and aluminum.
Roughening of surfaces by sandblasting, chemical etching or anodization has been commonly practiced to enhance adhesion of aluminum/polymer adhesive systems and to provide structural durability in humid or corrosive environments. However, when the bond itself must be electrically conductive, treatments that improve adhesion by producing a thick oxide layer, such as phosphoric acid or chromic acid anodization, are not suitable because of the poor electrical properties of the thick oxide layers. Sand blasted surfaces show some improvement in bond resistance stability, but still significant susceptability to hydrolysis and resistance increase in humid environments.
Accordingly, there is a need for new technology for fabricating reliable conductive interconnections between metal substrates, particularly aluminum substrates, and ground planes of printed circuit boards, especially when the resulting circuit board assembly is subject to humid environments over extended periods of time.